Centrifugal compressors have wide application in many industries. In the cryogenic air separation industry, centrifugal compressors are employed to compress the feed air entering the plant where it will be liquified and separated into its constituents. Centrifugal compressors are employed to compress product nitrogen gas and product oxygen gas. The energy required to compress these streams is a major cost in the plant operation. Thus even small improvements in the efficiency of the centrifugal compressors performing the compressions will have a significant effect on the economics of the process.
A conventional centrifugal gas compressor has a rotatable shaft on which is mounted an impeller having blades for ingesting gas in an axial direction. The impeller is rotated thereby accelerating and raising the pressure of the gas. The gas is expelled from the impeller in a radial direction with considerable velocity having a large tangential component. The gas enters a diffuser, where it turns to flow more radially outward, decelerates and continues to rise in static pressure. The diffuser may have vanes to guide the gas flow, or have no vanes, being vaneless, that is, an empty space. In applications where radial compactness is important and efficiency is paramount, a vaned diffuser, being more efficient, is preferred over a vaneless diffuser.
The gas leaves the diffuser with a kinetic energy equal to one to four percent of the energy imparted to the gas by the impeller. The gas velocity leaving the diffuser has both a radial component and a tangential (circumferential) component. A vaned diffuser discharges the gas with a smaller tangential velocity component than a vaneless diffuser.
Encircling the diffuser outlet is a housing with an internal channel having an opening aligned with the diffuser outlet. The channel accepts the gas flow discharging from the circumference of the diffuser. The channel walls direct the gas flow into a tangential direction to flow circumferentially in the channel. The channel accumulates and merges the flow into a single stream which leaves the channel through an outlet duct emerging from the channel housing. Typically the outlet duct emerges from the housing in a tangential direction to minimize flow losses. The outlet duct conveys the compressed gas flow to a pipe which conveys the compressed gas to the next process point.
To accumulate or collect the gas discharging from a vaneless diffuser, an accumulating channel known as a volute is typically used. A conventional volute comprises a housing having a cross section which increases progressively from almost zero area at an origin, known in the art as the tongue, to a maximum area at its outlet, known as the throat. The area schedule requirements result in a small tongue angle, such as 10.degree. as measured from the tangential direction. A volute is usable with a vaneless diffuser, which typically discharges flow with a large tangential component and small exit angle measured from the tangential direction. Thus the flow incidence angle relative to the tongue is low and incidence losses are small. A vaned diffuser typically turns the flow into a more radial direction and discharges flow with a smaller tangential component. Thus the flow from a vaned diffuser on entering a volute would have a greater incidence on the volute tongue with increased dissipation of flow energy.
To avoid incidence losses imposed by the volute tongue, an accumulating channel known as a collector is used with vaned diffusers. A conventional collector comprises a toroidal housing with a constant radial cross-section of sufficiently large area to serve as a plenum to minimize circumferential pressure variation in the collector.
The ordinary collector usually used with a vaned diffuser presents opportunity for improvement. At the circumferential locations at and near the collector origin, which is adjacent to the entrance to the emerging outlet duct, there is little circumferential flow in the collector. There the flow leaving the diffuser decelerates abruptly on entering the collector, with an attendant loss in total pressure. At circumferential locations approaching the end of the collector, that is approaching the outlet duct, considerable circumferential flow accumulates in the collector, and the flow leaving the diffuser accelerates on entering the collector. Subsequently this accelerated flow typically is decelerated, with an attendant loss in total pressure.
On the other hand, where an oversized collector can be accommodated, the flow from a diffuser on entering the collector will decelerate around the entire periphery of the collector, which will result in increased kinetic energy losses. Thus conventional collectors do not well preserve the kinetic energy remaining in the gas exiting vaned diffusers.